Along with the popularization of mobile communication as represented by cellular phones, the demand for a high frequency filter operated in several hundred MHz-several GHz has been increased. Among various filter technologies, the filter manufactured by using a bulk acoustic wave resonator is characterized by (1) high frequency, (2) compact, (3) excellent temperature characteristic, and (4) excellent electric strength.
The bulk acoustic wave resonator is a stacked resonator having a structure in which electrode films made of metal materials sandwiches an piezoelectric layer such as AlN and ZnO therebetween. The longitudinal resonant oscillation occurred by applying AC voltage to the stacked resonator is used. The structure of the bulk acoustic wave resonator is categorized by methods of confining the bulk acoustic wave in the piezoelectric film. Among them, there is a method that a cavity is provided on the bottom of the resonator thereby a diaphragm structure is formed. As the above mentioned technology for forming the diaphragm structure in the bulk acoustic wave resonator, there is a configuration that the piezoelectric layer is sandwiched between the bottom electrode layer and the top electrode layer, and the sandwich structure is supported on the cavity by an underlayer (cf. non-patent document 1).
As another technology for forming the diaphragm structure, there is a configuration that a sacrificial layer is patterned in an area for air gap on the support substrate, and a sandwich structure in which the piezoelectric layer is sandwiched between the bottom electrode layer and the top electrode layer is formed thereon, then, the sacrificial layer is removed to obtain an air gap on the bottom of the sandwich structure (cf. patent document 1).
As another diaphragm structure forming technology, there is a configuration that a concave portion in an area for air gap on the support substrate is previously formed, the concave portion is filled with the sacrificial layer, and a sandwich structure in which the piezoelectric layer is sandwiched between the bottom electrode layer and the top electrode layer is formed after smoothing, and then, the sacrificial layer is removed to obtain an air gap on the bottom of the sandwich structure (cf. patent document 2).
Further another technology for forming the diaphragm structure, there is a configuration that a sandwich structure in which the piezoelectric layer is sandwiched between the bottom electrode layer and the top electrode layer is provided in contact with the first surface of the support substrate, and an air gap including a first aperture opening at the first surface and a second aperture opening at the second surface at the position corresponding to the sandwich structure on the support substrate, respectively and having a generally vertical shape to the first surface being formed using Deep-RIE (cf. non-patent document 2 or patent document 3). According to the patent document 3, it is suitable for downsizing and can obtain a bulk acoustic wave resonator having an excellent resonance characteristic. Additionally it is described that since the cut surface of the support substrate is important to obtain a sufficient capability for the orientation of the piezoelectric layer, a Si(111) substrate is appropriate for the support substrate. It is also described that if a Si(100) substrate is used, the orientation of the piezoelectric layer is significantly deteriorated so that it is difficult to obtain an excellent resonance characteristic in the bulk acoustic wave resonator.
Patent document 1: Japanese Patent Laid-open No. 60-189307
Patent document 2: Japanese Patent Laid-open No. 2000-69594
Patent document 3: Japanese Patent Laid-open No. 2003-204239
Non-patent document 1: 1994 IEEE International Frequency Control Symposium pp. 135–138
Non-patent document 2: 2002 IEEE International Ultrasonics Symposium pp. 969–972)